The large majority of the total knees implanted today are either of the posterior cruciate retaining (CR) or posterior stabilized (PS) types. These have both functioned well clinically, but a number of disadvantages remain. Due to variations in surgical techniques and between patients, when using the CR type of total knee, it has been difficult to obtain an optimal tightness of the posterior cruciate ligament (PCL) throughout flexion, resulting in variable rollback patterns.
For the PS, considerably extra bone needs to be resected from the center of the femur to accommodate the intercondylar housing of the femoral component, for the cam and post mechanism. Damage can sometimes occur to the plastic post in the long term, anteriorly due to impacts at full extension, and posteriorly due to high flexion loading. In addition, in many PS designs, there is a tendency for overconstraint in rotation due to the dishing of the tibial bearing surfaces, in combination with the construct of the cam-post.
A disadvantage common to both CR and PS types, although more pronounced with CR designs due to their shallower tibial bearing surfaces, is a paradoxical motion in which the femur slides forwards on the tibial surface during the first half of the flexion range, rather than backwards which is the required motion. Also, uneven or jerky motion occurs in many cases. Artificial knees have included configurations that accommodate medial pivotal rotation to resemble the anatomical motion in the anatomic knee. However, these configurations may not incorporate a mechanism for achieving lateral rollback in flexion in combination with a relatively immobile medial side, or do not provide sufficient laxity about the neutral path of motion.
A knee joint system is needed that has a femoral component and a tibial component that have specific contours and surfaces that guide the knee joint into an average anatomic neutral path of motion during flexion-extension, while having sufficient laxity about that neutral path to accommodate different individuals and activities; and that can be used as both a CR and PS types.